1. Field of the Invention
This invention relates to a method of producing a single crystal gallium nitride (GaN) substrate and a single crystal GaN substrate utilized as a substrate of making laser diodes (LDs) and light emitting diodes (LEDs) composed of groups 3-5 nitride semiconductors.
2. Description of Related Art
Light emitting devices based upon group 3-5 nitride semiconductor include blue/green light emitting diodes and blue light laser diodes. Blue light LEDs have been sold on the market. But, LDs have not been on the market yet. Almost all of the conventional 3-5 nitride light emitting devices and laser diodes (LEDs, LDs) have been fabricated upon sapphire (α-Al2O3) substrates. Sapphire is a rigid and sturdy material. Sapphire excels in chemical and physical stability. Another advantage of sapphire is to allow GaN heteroepitaxial growth on it. Thus, GaN films, AlGaN films or InGaN films can be grown on sapphire substrates. Sapphire has been an exclusive, pertinent substrate for GaN type LEDs.
Sapphire, however, has some drawbacks as a substrate. Sapphire lacks cleavage. Sapphire is not a semiconductor but an insulator. GaN films or InGaN films grown on a sapphire substrate are annoyed by large lattice misfitting. Lattice misfitting means a difference of lattice constants between a substrate and a film. Sapphire belongs to trigonal symmetry group. Sapphire lacks three-fold rotation symmetry and inversion symmetry. Poor symmetry deprives sapphire of cleavage planes.
The use of sapphire substrates forces device makers to cut a processed GaN wafer into individual chips by mechanical dicing instead of natural cleavage. To dice a hard, sturdy, rigid sapphire plate mechanically into pieces is a difficult process, which decreases yield and enhances cost.
Noncleavage further induces a serious difficulty of making good resonator mirrors of laser diodes. The resonators are made by mechanical polishing, which raises the cost of LDs and declines the quality of the resonators.
Insulation is another weak point of sapphire. Insulating sapphire incurs a difficulty of n-electrodes. An insulating substrate forbids an LED from having an n-electrode on the bottom unlike an ordinary diode. An n-electrode is formed by etching away a top p-GaN layer, an active layer, revealing an intermediate n-GaN film on the sapphire substrate, depositing an n-metal electrode on the n-GaN film, and wirebonding the n-metal electrode with a lead pin. The etching for revealing the intermediate film and wirebonding are extra steps which are required for making an n-electrode on the on-sapphire device.
Current flows in a horizontal direction in the n-GaN film. The n-GaN film should be grown to a thick film for reducing electric resistivity of the n-GaN film. Extra steps and extra components raise the cost of fabrication.
Since two electrodes are formed on n- and p-films within a chip, an extra wide area is required for the chip. The wide, large chip raises cost up.
The third weak point of a sapphire substrate is lattice misfitting. Lattice misfitting induces high density dislocations into GaN epi-layers grown upon a sapphire substrate. It is said that GaN epi-layers of on-sapphire LEDs sold on the market should have 1×109 cm−2 dislocations.
Another candidate for a substrate is silicon carbide SiC, since lattice misfit between SiC and GaN is smaller than the GaN/sapphire misfit. A GaN grown on a SiC substrate turns out to have a similar high dislocation density to the on-sapphire GaN layers. SiC does not surpass sapphire as a substrate.
High dislocation density in GaN, InGaN, AlGaN epi-layers causes no problem in the nitride-type LEDs because of low current density. In the case of LDs having a narrow striped electrode and a narrow emission area, high density current would reproduce dislocations and the increased dislocations would shorten the lifetime of LDs. Non-cleavage, insulator and misfit are three serious drawbacks of sapphire substrates.
The best candidate for an ideal substrate for LDs is a gallium nitride (GaN) single crystal substrate. If a high quality GaN single crystal substrate were obtained, the problem of the lattice misfitting would be solved, because a device would take a GaN/GaN homoepitaxy structure.
A GaN crystal has cleavage planes {1-100}. Cleavability of GaN enables device makers to divide a processed GaN wafer into individual chips along cleavage planes. Cleavage lowers the difficulty and cost of chip separation. Resonator mirrors of LDs can be easily produced by natural cleavage. High quality resonators are formed by the cleavage.
GaN can be doped with n-type dopants or p-type dopants. Doping with an impurity can prepare a conductive GaN substrate. Since a low resistance n-type substrate is made by doping with an n-type dopant, an n-electrode can be formed at a bottom of an n-GaN substrate. Vertical electrode alignment enables an LD and an LED to reduce a chip size, simplify a device structure and curtail the cost.
However, GaN single crystals are not yielded as natural resources. Production of GaN single crystals is difficult. Manufacture of high quality GaN single crystal substrates with a practical size has been impossible till now.
It is said that ultrahigh pressure and ultrahigh temperature would realize production of a tiny GaN crystal grown from a mixture of melt/solid at thermal equilibrium. The ultrahigh pressure method is impractical. A wide GaN substrate cannot be made by the method.
Methods of making GaN substrate crystals by growing a thick GaN crystal on a foreign material substrate in vapor phase and eliminating the foreign material substrate had been proposed. The vapor phase method has been inherently a method for making thin GaN, AlGaN, InGaN films on a sapphire substrate. The vapor phase method was diverted from film piling to substrate production. The inherent vapor phase method is unsuitable for substrate production. Large inner stress and many dislocations appeared in the GaN films made by the vapor phase method. Large inner stress prevented GaN films from growing thick crystals sufficient for substrates. A GaN “substrate” is a final product of the present invention. A substrate of a foreign material, e.g., sapphire or GaAs is a starting base plate for making GaN. Two substrates should not be confused. For discriminating two kinds of substrates, the starting foreign substrate is here named “undersubstrate”.
The inventors of the present invention proposed an epitaxial lateral overgrowth method of growing a GaN via a mask on an undersubstrate in vapor phase ({circle around (1)}Japanese Patent Application No.9-298300, {circle around (2)}Japanese patent application No.10-9008).
In the concrete, the ELO method proposed by us was a method of preparing a GaAs undersubstrate, producing an SiO2 or SiN film on the GaAs undersubstrate, perforating many small windows regularly and periodically aligning with a short pitch (spatial period), growing a GaN film on the masked GaAs substrate in vapor phase for a long time, and eliminating the GaAs substrate. The ELO alleviates inner stress and dislocations. The preceding ELO method utilized sapphire as an undersubstrate, which may be called an on-sapphire ELO. But, the above ELO method made use of GaAs as an undersubstrate. The method of the present inventors is called here an on-GaAs ELO.
The inventors of the present invention have proposed a method of making a plurality of GaN substrates by homoepitaxially growing a thick GaN crystal upon a GaN substrate obtained by the former mentioned ELO method, making a tall GaN ingot and slicing the tall GaN ingot into a plurality of wafers ({circle around (3)}Japanese Patent Application No.10-102546).
The improved ELO gave a probability for making wide GaN single crystal substrates on a commercial scale.
The ELO made GaN crystals were plagued with high dislocation density. The ELO reduces dislocations at an early stage of the growth. During the long time growth, however, dislocations increase again. Bad quality prohibited the ELO-GaN substrates from being the substrates for producing nitride type laser diodes (InGaN-LDs). Production of high quality (long lifetime) LDs required lower dislocation density GaN substrates.
Mass production of devices requires wide GaN substrates which have low dislocation density and high quality in a wide area.
The inventors of the present invention proposed a method of making low dislocation density GaN substrate ({circle around (4)}Japanese Patent Laying Open No.2001-102307). The present invention is an improvement of the former method {circle around (4)}.
The method proposed by {circle around (4)} is now called “facet growth” method in short. The method reduces dislocations by forming three-dimensional facets and facet pits of e.g., reverse-hexagonal cones on a growing surface intentionally, maintaining the facets and pits, growing a GaN crystal without burying the pits, gathering dislocations by the facets to the bottom of the pits, and reducing dislocations in other regions except the pit bottoms.
Three-dimensional facet pits are otherwise reverse-dodecagonal cones built by facets. The facets comprise typical {11-22} and {1-101} planes.
The facet growth {circle around (4)} (Japanese Patent Laying Open No.2001-102307) proposed by the inventors grows a GaN crystal in vapor phase on the condition of making facets and maintains the facets without burying the pits of facets. The facets grow not in the c-axis direction but in a direction normal to the facets. The roles of facets and pits in the facet growth {circle around (4)} (Japanese Patent Laying Open No.2001-102307) are described with reference to FIG. 1 which shows a small part around a facet pit on a surface of a GaN crystal growing in the facet growth. In practice, many facets and facet pits appear on the surface. A vapor phase epitaxy method (HVPE, MOCVD, MOC or Sublimation) grows a GaN crystal on a substrate in a direction of a c-axis. The growth is a c-axis direction growth but is not a “C-plane growth” which has been prevalent in the conventional GaN growth. Facets grow in directions normal to the facets.
Conventional C-plane growth methods grow a GaN film epitaxially on a substrate by maintaining a smooth C-plane surface. Produced GaN crystals have poor quality of high dislocation density, for example, 1010 cm−2. Our new facet growth method intentionally makes facets and pits, maintains the facets and reduces dislocations by make the best use of the function of facets of gathering dislocations into pit bottoms.
The facet growth produces plenty of reverse hexagonal cone pits 4 on the growing GaN surface. FIG. 1 shows a single one of many pits. Six slanting planes are low index facets 6 of {11-22} or {1-101} planes. A flat top surface 7 outside of the pit 4 is a surface of C-plane growth. In the pit, the facet grows inward in the direction of a normal standing on the facet as shown by inward arrows. Dislocations are swept to corner lines 8 by the growing facet. Dislocations are gathered on the six corner lines 8.
The dislocations swept to the corner lines 8 slide down along the corner lines to the bottom of the pit. In practice, the dislocations do not fall along the corner lines 8. The growth raised the facets, the corner lines and the pit bottoms at a definite speed. Sliding dislocations along the rising corner lines centripetally move inward in horizontal directions. Finally, the dislocations attain to the center of the pit just at the time when the pit bottom rises to the height of the dislocation. Then, dislocations are accumulated at the bottom of the pit. The number of the dislocations on the facets is reduced by the accumulation of dislocations at the bottom.
Proceeding of the facet growth sometimes forms planar defects 10 following the corner lines 8 by storing the swept dislocations at the corner lines. The planar defects are six planes with sixty degree rotation invariance corresponding with the hexagonal symmetry of GaN. The width of the planar defects 10 is equal to the diameter of the pit 4. The six planar defects 10 cross at a vertical extension of the pit bottom. The crossing line forms a linear defect assembly 11 having highly concentrated dislocation. Ideally all the dislocations initially existing in the pit are swept to and are accumulated at the pit bottom. The other parts lose dislocations and become low dislocation density single crystals. This is the dislocation reduction method proposed by {circle around (4)} (Japanese Patent Laying Open No.2001-102307).
Finally, the majority of dislocations are concentrated to the pit center. The operations of the facets reduce dislocation density in the regions included within the projection of the pits.
There are some problems in the new facet growth method proposed by {circle around (4)} which makes facet pits at random spots accidentally, maintains the facet pits, grows a GaN crystal without burying the pits, and concentrates dislocations to the bottoms of the facet pits.
Though the facets gather dislocations to the pit bottoms, dislocations are not concentrated fully into a narrow, restricted spot. For example, when 100 μm φ pits are yielded, some pits can concentrate dislocations to a small spot at the bottom of a several micrometer diameter but other pits have about 50 μm φ hazy dislocation dispersion region of medium dislocation density near the bottom.
FIG. 3 demonstrates the occurrence of the hazy dislocation dispersion. FIG. 3(1) shows that a c-axis crystal growth (arrows) moves facets 16 inward, dislocations on the facets are carried by the facets 16 in horizontal directions (shown by horizontal lines) to the pit bottom and the bottom has a linear dislocation bundle 15. But, repulsive forces release once gathered dislocations outward. FIG. 3(2) shows that the once concentrated dislocations 15 are diffusing from the bottom to the facet 16 of a pit 14. Occurrence of hazy dislocation dispersion is a drawback of the facet growth of {circle around (4)}.
If the pit size is enlarged for widening the area of good quality portions, the area of the hazy dislocation dispersion further dilates. The reason is supposed that enlargement of a pit size increases the number of the dislocations gathered at the bottom and the number of the dislocations released from the bundle.
The inventors think that the release of dislocations from the pit centers results from repulsion acting between concentrated dislocations. Unification of pits incurs disorder of dislocations and expansion of the hazy dispersion of dislocations. Excess concentration induces the hazy dislocation dispersion.
The hazy dislocation dispersion has about 2×107 cm−2 dislocation density which has dependence to positions. Such a high dislocation density GaN substrate is insufficient for making laser diodes (LDs) of a satisfactory lifetime. A long lifetime of LDs requires to reduce dislocations down to one twentieth ( 1/20) of the current value (2×107 cm−2), that is, to 1×106 cm−2.
Another problem is the existence of planar defects 10 produced under the corner lines of pits as shown in FIG. 1(b). The planar defects are radially arranged with 60 degree rotation symmetry. Facets assemble dislocations at pit corner lines. Without progressing to the center bottom, the assembled dislocations form planar defects 10 by dangling from the corner lines. A planar defect can be considered as an alignment of dislocations in a plane. The planar defects are another problem of the conventional facet growth method. Sometimes a slide of crystal planes occurs on both sides of the planar defect.
Besides the 60 degree rotation symmetric planar defects, 30 degree rotation symmetry planar defects sometimes appear in dodecagonal pits on a growing surface. Planar defects appear as dislocation arrays on the surface of the growing substrate. Planar defects are a serious hindrance to produce long lifetime LDs. Prolongation of LD lifetime requires reduction of the planar defects.
Another problem is distribution of defects. Dislocation reduction of the facet growth method makes use of facet pits accidentally and randomly appearing on a facet growth. Positions of pits are not predetermined. Numbers of appearing pits are also not programmable. Positions, numbers, shapes and sizes of appearing pits are all stochastic, random, accidental variables which are unpredeterminable, unprogrammable, uncontrollable. It is a problem that the positions of pits are uncontrollable.
If a plurality of laser diodes were fabricated upon a GaN substrate having random distributing planar defects, emission stripes of active layers of the laser diodes would accidentally coincide with the defect assemblies which occupy random spots on the GaN substrate. In the case of coincidence of the active layer with the defect bundles, important emission layers are plagued by the defect assemblies. Large current density driving current would invite rapid degeneration on emission stripes from the inherent defects of the laser diodes.
Uncontrollability of the positions of pits would decrease the yield of manufacturing laser diodes on the substrate.
Manufacturing GaN substrates for making laser diodes thereon requires enhancement of yield through controlling the positions of dislocation bundles on the GaN substrates. It is important to control the positions of dislocation bundles not to collide with emission stripes of laser diode chips on the GaN substrates.
Three problems have been described for long lifetime laser diodes.    (1) Reduction of hazy dislocation diffusion from the pit center composed of facets,    (2) Extinction of planar defects at the bottoms of the pits composed of facets,    (3) Controlling of positions of the pits made of facets.
The present invention aims at overcoming the difficulties of these three problems.
Before fundamental principles of the present invention are described, the three problems are clarified further.
A problem of the previous facet growth maintaining facet pits is a state of an assembly of dislocations. Propagation of dislocations on the facets in the pits sweeps and concentrates many dislocations to the center of the pit. The state of dislocation assemblies is unstable, which is a serious problem.
Repulsive force occurs between two dislocations of the same sign Burgers vectors. The repulsive force tends to release bundles of the once concentrated dislocations by giving the dislocations centrifugal forces. The dislocations diffuse outward by the repulsion. The diffusion yields hazy dispersion of dislocations in the vicinity of the dislocation bundles. The hazy dislocation dispersion is a problem.
The reason of making the hazy dislocation dispersion is not clear enough yet for the inventors. One reason is stress concentration due to the dislocation convergence. A plurality of pits are often coupled into a bigger pit during the growth. Coupling pits disturbs the arrangement of dislocations. Perturbation of the dislocation arrangement is another reason of the hazy dislocation diffusion occurring.
The number of assembled dislocations to the dislocation confluence increases. The increase of dislocations enlarges the hazy dislocation dispersion. Another reason is an increase of dislocations by the coupling of pits.
While dislocations gather to the center of the pits composed of facets, corner lines between neighboring facets yield six planar assemblies of dislocations hanging from the corner line, which lie along 6 radii which coincide with each other by 60 degree rotation. The planar defects hanging on the corner lines are generated by the facets sweeping dislocations to the six corner lines of hexagonal pits.
When the unification of pits enlarges a pit size, the number of the dislocations centripetally converging to the center increases, which enhances further the size of the planar defects. This is another drawback of the previous facet growth.
The positions of pit appearing are random, stochastic and accidental matters. Pits appear at random spots by chance. The positions of the facet pits are uncontrollable, stochastic and random.
When optoelectronic devices are produced upon a GaN substrate with the wide hazy dislocation dispersion, random dislocation assemblies fluctuate qualities of the devices, which decreases the yield of the device production.
Next, the fourth problem is,    (4) Occurring of microcracks at interfaces between the voluminous defect accumulating regions (H) and the low dislocation single crystal regions (Z) due to difference of thermal expansion.
The voluminous defect accumulating regions (H) take various crystal structures. Polycrystalline voluminous defect accumulating region (H) induces microcracks at the interfaces. The microcracks are caused by random difference of thermal expansion in a polycrystal. Thus, polycrystalline voluminous defect accumulating region (H) is not best.
Single crystal voluminous defect accumulating region (H) with slanting axes or slantingly inverse axes also incurs microcracks at the interfaces. The reason why the microcracks occur is also thermal expansion anisotropy different from the other single crystal parts.
Further, the fifth problem is,    (5) Difficulty of stably producing single crystal regions with inversed orientations in GaN wafers at low cost.
Properties of the voluminous defect accumulating region (H) depend upon accidents. It is difficult to always form orientation-inverse voluminous defect accumulating regions (H) on masked undersubstrates. For example, in the case of forming an SiO2 mask on a sapphire undersubstrate, sometimes polycrystalline voluminous defect accumulating region (H) grows on the mask. The other times slanting orientation single crystal voluminous defect accumulating region (H) grows on the mask. Once formed voluminous defect accumulating region (H) disappears halfway. Sometimes voluminous defect accumulating regions (H) of orientation-inverse single crystals mixed with polycrystals are born on the mask.